Porous metallic product and method for making same

ABSTRACT

This invention pertains to a product and a method for making the product. The product is a lightweight solid porous metallic product containing small solid spheres having a coating of a primary alpha phase thereon disposed in a solid metal alloy eutectic matrix. The method includes the steps of mixing the hollow rigid spheres and a metal alloy, which metal alloy can be preheated to render it molten, in order to form a dispersion of the spheres distributed in the molten alloy; initially cooling the dispersion to render it semi-solid whereby the spheres are coated by a solid and the coated spheres are disposed in the semi-solid mixture of the solid and liquid; and finally cooling the sphere-containing semi-solid mixture to a temperature at which the sphere-containing semi-solid mixture becomes solid and the product is formed.

FIELD OF THE INVENTION

This invention pertains to the field of porous metallic objects and preparation thereof that is characterized by the use of small hollow spheres in a semi-solid mixture in a metal alloy.

DESCRIPTION OF THE RELATED ART

Lightweight or porous metallic materials and structures are made in honeycomb and foam core forms. Other examples are linear cellular metals and metal wire trusses. Most of these materials are made in simple shapes such as two-dimensional panels and one-dimensional bars and rods. Making these into complex three-dimensional shapes is very difficult and cost intensive. This is because of the nature of the processing routes for these materials. These materials are expensive to fabricate because they require several processing steps. Honeycomb structures are made by epoxy joining or brazing together a honeycomb arrangement made by folding thin metal ribbons. This forms the honeycomb core to which are joined face panels to make a sandwich structure. Foam materials are made through a powder metallurgy route, which involves mixing volatile materials with the metallic powder and high temperature processing. The foam core is also joined to face panels to make a sandwich structure. Linear cellular metal structures are made by hydrogen reduction of extruded oxide structures at high temperature. Metal wire truss structures are made by liquid phase sintering of the truss joints. To obtain the truss configuration, a complex weaving operation is required. Some of these materials have good stiffness but lack in other mechanical properties, such as tensile and compressive strengths. Because of their high fabrication cost, these lightweight materials and structures have been targeted for high-end applications, such as aerospace, propulsion and ballistic protection. Many of these materials are made abroad and availability thereof is severely restricted.

OBJECTS AND BRIEF SUMMARY OF THE INVENTION

It is an object of this invention to make lightweight or porous metallic products using small, hollow ceramic or metallic particles to reduce density thereof.

It is another object of this invention to make net-shape lightweight products that do not require substantial machining.

Another object of this invention is lightweight, porous metallic products that retain greater than the expected amount of their physical properties, i.e., lower density, higher relative strength and higher relative stiffness.

Another object of this invention is a method for preparing the lightweight metallic products, the method being characterized by the use of semi-solid metal alloy.

Another object of this invention is a method for preparing lightweight metallic products, the method being characterized by cooling a liquid (L) metal alloy to a semi-solid or semi-liquid state.

Another object of this invention is a method of making a lightweight metallic product, the method being characterized by a short holding period in the semi-solid or semi-liquid state in order to stabilize the solid alpha (α) and the liquid (L₁) phases.

Another object of this invention is a method of using semi-solid region in eutectic, peritectic and monotectic metallic alloy systems.

These and other objects of this invention can be attained by preparing a lightweight, net-shape product having better than expected physical properties by mixing small ceramic and/or metallic hollow spheres with a metal alloy, such as eutectic type, heating the mixture to liquify the alloy whereby the spheres are coated with a solid primary alpha (a) phase surrounded by liquid L, phase, and solidifying the resulting mass whereby the hollow spheres have a coating of the solid primary alpha phase surrounded by the solid metal alloy of eutectic composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a typical binary eutectic phase diagram for metallic alloys.

FIG. 2 is a schematic illustration in cross-section of a porous metal alloy product showing dispersed hollow spherical particles having a solid coating of a primary or alpha phase on the particles disposed in a solid metal alloy of eutectic composition.

FIG. 3 is a graph of a volume fraction φ and scaled separation distance showing separation distance between voids as a function of void volume fraction.

FIG. 4 is a graph of volume fraction φ and Modulus of Elasticity or stiffness, in MPa.

FIG. 5 is a graph of ratio of inside to outside diameter of a hollow ceramic spherical particle and normalized density of a sphere.

DETAILED DESCRIPTION OF THE INVENTION

This invention pertains to a method for making a lightweight porous metallic product and to the product itself for eutectic, peritectic and monotectic systems, although only the eutectic case is discussed herein.

The invention is a novel approach for making under-dense or lightweight or porous metallic materials, especially into a net shape. The method involves mixing nanoscopic or microscopic hollow spheres into a metal alloy of at least two components during processing. The spheres are typically ceramic since most ceramics have a higher melting point than a metallic alloy. The spheres, however, can be non-ceramic, such as metallic, wherein the metal the sphere is made of has a melting point that is higher, and preferably substantially higher, such as several times higher, than the melting point of the matrix metal alloy and is generally immiscible in the matrix metal alloy. The hollow spheres can be ceramic coated with a metal or they can be metallic coated with a ceramic. This creates a dispersion of nanoscopic or microscopic spherical voids or pores in a fully dense metal matrix thereby reducing the effective density of the composite. To be effective, the dispersion has to be uniform and clustering or agglomeration has to be avoided. Thin-walled hollow nanospheres or microspheres have a low specific gravity, preferably less than one. The bulk density of the metallic alloy will be lowered because the hollow spheres create free volume or space. Additionally, the uniformity of distribution of the small rigid hollow spheres in the metallic matrix, at the same time, maintains the integrity of the product.

The molten metallic phase is of any metal component and any alloying component. At least one metal and at least one alloying component are used. Of particular interest here are the commercial alloys, especially the alloys of the light metals such as magnesium, aluminum and zinc, but not limited to these. Some of the alloys of interest include, but are not limited to AZ-91-D, AM-50-A, AM-60-B, AS-41-B, A356, A357, 319, A390, Zamak 3, and ZA-8. The spheres are tiny, hollow but rigid particles and include engineered and non-engineered particles, such as fly ash which is a by-product of flue gas resulting from the coal-burning process. The hollow nanospheres and microspheres can be ceramic or metallic and typically range in size from 10 to 100,000 nm, more typically from 100 to 10,000 nm. Since majority of the spheres must remain solid and hollow during processing, melting point thereof is in excess of the melting point of the surrounding metallic matrix. In order to avoid reactivity with the surrounding metallic matrix, the spheres are typically inert thereto.

The invention is described as adding tiny hollow spheres to a metallic alloy base material. The sphere-alloy mixture is fabricated into a desired product by shaping the mixture with one of several low cost, net-shape processing routes. Net-shape processing of metallic alloys can involve metal casting or powder metallurgy. Investment casting, die casting and semi-solid molding are a few of the metal casting routes. Metal injection molding, hot isostatic pressing and hot extrusion are a few of the powder metallurgy routes. Solid freeform fabrication or layered manufacturing is another near net-shaping technology and can involve melting and solidification of sequential weld pools or sintering and consolidation of powder particles. By the layered manufacturing technology, very complex geometries, not possible by conventional means, are fabricated. Net-shape products or parts require little of no finishing. The advantages of net-shape forming include, but are not limited to, low material waste, high throughput, high volume and low fabrication cost.

Critical to the success of the invention is to ensure uniform dispersion of the small hollow spheres and the base materials, minimize problems due to density differences between the spheres and the metallic phases, ensure integrity of the additives during mixing that the hollow spheres do not collapse and remain intact during processing, ensure uniform filling of the mold or any other cavity by the mixture or slurry without gross segregation. Finally, control of flow of the rheological mixtures is critical and wetting, high temperature reactions and dissolution issues should be considered.

The addition of hollow microspheres or nanospheres to the metal matrix serves a dual purpose: it lowers the density and increases the specific strength and stiffness of the product. The porosity and voids created by the hollow additives reduce the effective density of the composite aggregate. The hollow additives also act as particulate reinforcements in which case, while the absolute yield strength decreases due to the existence of pores, the effective yield strength of the composite aggregate increases due to constraint of plasticity flow offered by the hollow spheres. This is the case if it is assumed that the hollow additives have rigid, strong walls and do not deform under load. The same reasoning that is applied to precipitation hardening or strengthening can be applied to the spherical reinforcement, i.e., the reinforcements, whether as small, hard precipitates or small, hollow spheres, impede dislocation flow thereby achieving improved strength. Additionally, if the material of which the hollow additives are made of has higher stiffness than the matrix material, then the effective stiffness, calculated from the rule of mixtures, of the composite aggregate should increase. Table 1, below, gives the Elastic Modulus and density of several ceramics which will affect properties of the final porous metal product. Except for silica and boron nitride, most ceramics have a higher Elastic Modulus than most engineering metals whose stiffness ranges from 40 to 200 GPa. Compared to the lightest engineering metal, i.e., magnesium, which has a Modulus of Elasticity of 45 GPa, all ceramics listed in Table 1 have higher values. Hence, the specific stiffness of the composite aggregate can be expected to increase with the addition of ceramic microspheres or nanospheres. TABLE 1 Physical Properties of Some Ceramics Ceramic Density, g/cc Elastic Modulus, GPa silica 2.2 73 alumina 3.7 300 silicon carbide 3.1 410 aluminum nitride 3.3 330 boron nitride 1.9 74 silicon nitride 3.3 310

More specifically, in connection with an embodiment of this invention, porous product and the method for making it can be better described and understood by reference to FIG. 1, a eutectic phase diagram of a two-component metal alloy of a metal component “A” and an alloying component “B.” Component “A” of the alloy is the lighter component and is typically present in a weight amount exceeding 50%. It is possible, however, for component “A” to be the major component although in an amount below 50% if the alloy is one of more than two components. The phase diagram is a typical graph of weight composition “C” on the “x” axis versus temperature “T” on the “y” axis. The phase diagram of FIG. 1, illustrates presence of a liquid phase (L) above the liquidus lines 10, 11; presence of a solid primary or proeutectic phase alpha (α) at the left side of FIG. 1 bounded by solidus line 12 and solvus line 14; presence of solid phase beta (β) or proeutectic β at the right side of FIG. 1 bounded by solidus line 16 and solvus line 18; presence of a semi-solid or semi-liquid region of solid alpha (α) phase and liquid (L₁) i.e., α+L₁, at the left side of FIG. 1 bounded by lines 10, 12 and 20, which segment line 20 extends horizontally along the eutectic reaction temperature T_(eu) from point 22 to eutectic concentration point 24. The phase diagram of FIG. 1 also includes another semi-solid or semi-liquid region, that is of less interest herein, of solid beta (β) phase and liquid L₂, i.e., β+L₂, at the right side of the phase diagram of FIG. 1 bounded by lines 11, 16 and 26, which segment line 26 extends from point 25 on the eutectic reaction temperature T_(eu) to the eutectic concentration point 24. Also depicted on the phase diagram of FIG. 1 is the solid eutectic of alpha and beta phases (α+β) defined by lines 14, 18 and the eutectic reaction temperature line formed by segments 20, 26.

As illustrated in FIG. 1, the method typically includes the steps of mixing the hollow spheres and metal alloy chips to form a dispersion point “a”, melting the metal alloy, initial cooling, and final cooling. It should be understood that the method includes variations or embodiments which do not strictly adhere to the given steps. For instance, the mixing step can be carried out by mixing solid hollow spheres with solid metal alloy chips, as shown at 28, or else a melted alloy can be used and be mixed with the spheres in order to uniformly disperse the spheres in the molten metal alloy matrix or the spheres can be added at a later stage of the initial cooling into the semi-solid region or in any other way. Whichever way it is done, what is ultimately desired at this point is a dispersion of the hollow spheres in the metal alloy, shown in FIG. 1. The step of melting can take place resulting in a molten alloy of given composition, with the spheres dispersed therein, as shown at point “b” on the phase diagram of FIG. 1, where a schematic of the molten mixture is shown at 30. At temperature “b,” the spheres are in solid state. Typically, the mixture at point “b,” is cooled along vertical line 25 to point “c” within the semi-solid region α+L₁ where the spheres become coated with solid primary or proeutectic alpha phase. At point “c,” the status of the mixture is illustrated by depiction 34 in FIG. 1 where the mixture is shown as being composed of solid spheres 40 coated with the solid primary or proeutectic alpha phase 42 disposed in the liquid metal L, 41. Volume fraction of the liquid phase L, in the semi-solid region can be determined by drawing a horizontal line 32 across vertical line 25 at point “c”, which identifies the hypothetical cooling path of the mixture. Line 32 is composed of segment 36, to the right of vertical line 25 and segment 33, to the left of the vertical 25. The ratio of line 36 to line 32, which is the sum of lines 33 and 36, gives volume fraction of the solid alpha phase, which forms around the spheres, whereas the ratio of line 33 to line 32, yields volume fraction of the liquid L₁.

Of course it should be understood that as the mixture 30 is cooled from point “b” along the vertical line 25 to point “c,” some solid alpha phase will appear as soon as liquidus line 10 is crossed. At a temperature of T₁, for instance, composition of the alpha (a) phase is determined at point 51 and composition of the liquid L₁ is determined at point 52. So, as the temperature of mixture 30 is reduced from point “b” to point “c,” the composition of the alpha phase and the liquid phase in the semi-solid region will change after crossing liquidus line 10 and as the temperature is gradually reduced to point “c.” Hypothetically, one can visualize the process being conducted by formation of a thin concentric coatings, of solid alpha phase around the hollow spheres, with the coated spheres residing in the liquid L₁ phase. Composition of the alpha phase in the concentric coating is that determined by the solidus line 12 and composition of the liquid L₁ phase is determined by the liquidus line 10.

Above discussion is predicated on the assumption that cooling proceeds along the path of the vertical line 25 and enough liquid L, is present to facilitate filling of a mold cavity. However, if cooling proceeds along a vertical line through the eutectic composition point 24, it should be understood that there will not be any liquid L, to facilitate filling of mold cavity.

Depiction 34 shows rigid spheres 40 coated with solid primary or proeutectic alpha phase 42 disposed in the liquid matrix L₁. Not all spheres are coated with the alpha phase and the coating thereon may not be of a uniform thickness. Also, some primary of proeutectic alpha phase can form on impurity surfaces in the liquid L₁. It is desirable to have a uniform thickness of alpha phase on the spheres since the alpha phase provides a protective coating on the spheres and thus insulates them from the molten metallic matrix, which may be reactive therewith. The coating thickness can vary widely and can be uniform or non-uniform. Depending if the spherical particle is nanoparticle or microparticle and depending upon the selected alloy composition, the coating thickness is typically within the range 1 to 100% of sphere radius, more typically in the range 10 to 50% of sphere radius.

Injection of the semi-solid mixture can be made into a mold at temperature “b” and then cooling down along line 25 all the way into the solid region α+β to room temperature, or any other desired temperature. This scenario is possible but is not typical of processing since formation of the solid phase is brief and its deposition on the spheres is not entirely adequate. Deposition of the alpha phase on the spheres is governed by the fact that the spheres provide nucleation sites for the alpha phase and adequate time should be devoted for formation of the alpha phase and its stabilization. In this sense, a holding period of less than a few minutes, such as less than about 1 minute, is typically provided to allow the alpha phase to grow and stabilize on the sphere surface and form a coating.

So, whether the mixture is cooled slowly initially to a point within the semi-solid region or from a point above the liquidus line 10 quickly into the semi-solid region above the eutectic reaction temperature, denoted α+L₁ in FIG. 1, what results is depiction 34 wherein the solid spheres are surrounded by the primary proeutectic alpha phase disposed in a liquid metal matrix. The spheres can have a continuous coating of the primary phase thereon but the coating can also be discontinuous. Coating thickness can vary greatly, depending on many variables, including the relative proportion of the alpha phase, proportion of the spheres, if a holding period is employed, and others. Typically, coating thickness is in the range of 1 to 100%, more typically 10 to 50% of sphere radius.

Since too much solid alpha phase in the semi-solid makes it difficult to push the semi-solid mixture into a mold cavity and since too much liquid in the mixture weakens the product too much, it has been estimated that the area of operation for purposes herein, is about one half of the horizontal extent in the semi-solid region between points 22 and 24, more typically about one-third of the central region.

One of the final steps of the method is cooling to a low temperature, such as room temperature, whereas the final product 36 is formed, shown in FIG. 1. As shown, the final product 36 is all solid with the hollow spheres 40 coated with primary proeutectic alpha phase 42 disposed in the eutectic 44 of phases α and β. This is its form at point “d” and remains that at below the eutectic temperature T_(eu).

It should be realized that whether the mixture is cooled to point “c” from point “b” and then it is moved into a mold cavity or quickly cooled from point “b” to point “d” and from there it is moved into a mold cavity, determines whether liquid will be present to facilitate mold cavity filling. It should be apparent that it is only in the semi-solid region of α+L₁, that the mold cavity can be filled, although the semi-solid region of β+L₂ can also be used, depending on properties of the alloying element “B.” Cooling through point 24, as discussed, will not be accompanied by any liquid, unless independently provided. Although alpha and beta compositions in the eutectic α+β change slightly, as evidenced by the solvus lines 14, 18 on the phase diagram of FIG. 1, the compositions of the alpha and beta phases in the eutectic remain essentially that as determined by points 22 and 25, respectively. It is only if cooling is extremely slow that respective compositions are realized below the eutectic temperature.

As should be apparent, as soon as the eutectic temperature T_(eu) is crossed, the mixture solidifies into depiction 36 which has solid spheres 40 coated by the solid primary proeutectic alpha phase 42 and the coated spheres are distributed in the solid eutectic 44 of α+β of average composition determined by point 24. The primary proeutectic alpha phase is usually colorless or featureless under microscope but α+β eutectic is of a different microstructure and usually appears lamellar under the microscope. So, the product that is finally obtained, contains solid spheres 40 coated with the solid primary proeutectic alpha phase 42 of a composition determined by point 22 and the same alpha phase of composition 22 and the beta phase by point 25 disposed in the solid eutectic 44 of α+β.

When the above preparation procedure is followed, a product is obtained that has physical properties that exceed properties that can be predicted on the basis of the law of physical mixtures. On the basis of the method discussed herein, the resulting product has physical properties on the order of about 70% of the bulk properties versus about 50% when considered on the basis of physical mixtures containing a metal alloy and the small or tiny metal or ceramic hollow spheres.

Although preparation of a porous metallic product rich in component “A” has been demonstrated, it should be understood that a porous metallic product can also be prepared rich in component “B” by following a cooling path indicated by line 38 in FIG. 1. If this is done, then component “B” should be in general, lighter than component “A” and have other desired attributes.

Although, hypothetically, any rigid hollow spheres can be used herein to lighten a metal alloy, realistically speaking, only the lighter metal alloys of the lighter structural kind, such as magnesium, aluminum, zinc and even titanium, can especially benefit from the invention herein. Lightness of the starting metal is of import since it is unlikely that one would gain much by lightening a heavy metal.

While presently preferred embodiments have been described of the novel lightweight porous metallic product and the method for its preparation, persons skilled in this art will readily appreciate that various additional changes and modifications can be made without departing from the spirit if the invention as defined and differentiated by the claims that follow. 

1. A method for preparing a lightweight porous metallic product comprising the steps of (a) mixing rigid, hollow and solid spheres with a metal alloy in order to obtain a dispersion of the spheres in a semi-solid alloy; and (b) cooling the semi-solid alloy containing some liquid and the solid coated spheres to a solid.
 2. The method of claim 1 which includes the step of heating the metal alloy to a temperature at which it is in a liquid state and the spheres remain solid, rigid and hollow.
 3. The method of claim 2 wherein the alloy is composed of at least one metal and at least one alloying component and the spheres are selected from the group consisting of metallic and ceramic spheres.
 4. The method of claim 3 wherein size of the spheres is in the range of 10-100,000 nm in diameter; the sphere coating thickness is in the range of 1-100% of sphere radius; and volume fraction of the spheres relative to the alloy is 10-90%.
 5. The method of claim 3 wherein size of the spheres is in the range of 100-10,000 nm in diameter; the sphere coating thickness is in the range of 10-50% of average sphere radius; and volume fraction of the spheres relative to said alloy is 30-70%.
 6. The method of claim 5 wherein said spheres are inert to said molten alloy.
 7. The method of claim 6 wherein said alloy is an alloy of a metal “A” and an alloying component “B” having a phase diagram characterized by a liquid region at upper extremity thereof, a primary a phase at the left thereof where composition of “A” is in the majority, a semi-solid region below the liquid region above eutectic temperature and defined by liquidus and solidus lines, and a solid eutectic region below the eutectic temperature.
 8. The method of claim 7 wherein the primary a phase at the left side of the phase diagram is defined by solidus line at the upper extremity and a solvus line at the right side thereof; and the solid eutectic region is defined at its left side by the solvus line, by another solvus line at its right side, and by the eutectic temperature at its upper extremity; said initial and final cooling steps are conducted along a vertical line passing through the semi-solid region.
 9. The method of claim 8 wherein the vertical line crosses over less than one-half of the horizontal central extent in the semi-solid region.
 10. The method of claim 8 wherein the vertical line crosses over less than one-third of the horizontal central extent in the semi-solid region.
 11. A method for preparing a lightweight, porous and net-shape metallic product comprising the steps of (a) mixing rigid and hollow spheres with a metal alloy in order to obtain a dispersion of the spheres in the molten alloy; (b) initially cooling the dispersion to render it semi-solid whereby the spheres are coated with a solid phase and the coated spheres are dispersed in the liquid metallic material; (c) filling a mold cavity with the solid phase coated spheres dispersed in a molten metallic material; and (d) finally cooling the solid phase coated spheres dispersed in the molten material to a solid state in the mold vavity.
 12. The method of claim 11 which includes the step of heating the metal alloy to a temperature at which it is in a liquid state and the spheres remain rigid, hollow and solid.
 13. The method of claim 12 wherein the alloy is selected from the group consisting of magnesium alloys, aluminum alloys, zinc alloys and other lightweight alloys; and the spheres are selected from the group consisting of metallic, ceramic and coated composite spheres.
 14. The method of claim 13 wherein size of the spheres is in the range of 10-100,000 nm in diameter; the sphere coating thickness is in the range of 1-100% of average sphere radius; and volume fraction of the spheres relative to the alloy is 10-90%.
 15. The method of claim 13 wherein size of the spheres is in the range of 100-10,000 nm in diameter; and the sphere coating thickness is in the range of 10-50% of average sphere radius.
 16. The method of claim 15 wherein the spheres are inert to the molten alloy.
 17. The method of claim 16 wherein the alloy is an alloy of a metal “A” and an alloying component “B” having a phase diagram characterized by a liquid region at upper extremity thereof, a primary a phase at the left thereof where composition of “A” is in the majority, a semi-solid region below the liquid region above eutectic temperature and defined by liquidus and solidus lines, and a solid eutectic region below the eutectic temperature.
 18. The method of claim 17 wherein the primary a phase at the left side of the phase diagram is defined by solidus line at the upper extremity and a solvus line at the right side thereof; and the solid eutectic region is defined at its left side by the solvus line, by another solvus line at its right side, and by the eutectic temperature at its upper extremity; said initial and final cooling steps are conducted along a vertical line passing through the semi-solid region
 19. The method of claim 18 wherein the vertical line crosses over less than one-half of the horizontal central extent in the semi-solid region.
 20. The method of claim 18 wherein the vertical line crosses over less than one-third of the horizontal central extent in the semi-solid region.
 21. A product comprising hollow rigid spheres having a solid coating thereon disposed in a solid metal alloy eutectic.
 22. The product of claim 21 wherein said solid coating on said spheres is primary proeutectic alpha phase and volume fraction of said spheres relative to said alloy is 10-90%.
 23. The product of claim 22 wherein said alloy is composed of at least one metal component and at least one alloying component.
 24. The product of claim 23 wherein said metal alloy eutectic is α and β phases.
 25. The product of claim 24 wherein said primary alpha phase coating is featureless in the microscope and is of different microstructure than is the α+β eutectic mixture.
 26. Lightweight porous net-shape product having physical properties that are better than that of bulk product physical properties comprising ceramic hollow spheres having a solid coating thereon disposed in a solid metal alloy eutectic.
 27. The product of claim 26 wherein said solid coating on said spheres is primary proeutectic alpha metal alloy phase and volume fraction of said spheres relative to said alloy is 30-70%.
 28. The product of claim 27 wherein said alloy is selected from the group consisting of magnesium alloys, aluminum alloys and zinc alloys.
 29. The product of claim 28 wherein size of said spheres is 100-10,000 nm; said sphere coating thickness is 10-50% of average sphere radius; and volume fraction of said spheres relative to said alloy is 30-70%.
 30. The product of claim 29 wherein said alloy eutectic is α and β phases and wherein said primary alpha phase is of a different microstructure than said eutectic mixture. 